This invention relates to gravity door hinge which can close a door automatically by its own weight.
Several door hinges of the above type have conventionally been proposed such as that disclosed in Japanese Utility Model Publication No. 1420/1951 and Japanese Patent Publication No. 19394/1977.
In the gravity door hinge according to Publication No. 1420/1951, the door hinge includes a fixed member and a rotational member, both members having cylindrical parts. The cylindrical part of the fixed member is provided with an inclined cam surface on its upper end surface, while the cylindrical part of the rotational member is provided with an inclined cam surface corresponding to the above cam surface on its lower end surface. An axial rod is inserted through the cylindrical parts of the fixed and rotational members so that the two inclined cam surfaces abut each other at the middle of the axial rod.
With the thus constructed door hinge, when a door to which the rotational member is secured is opened by pushing or pulling on the side opposite to the secured side, the door is raised by the cooperation of the two inclined cam surfaces. Because the rotational member is not secured to the door at the center of gravity of the door but at a position to one side of the door, a large bending moment is generated by the off-center load of the door, during the rotating and rising motion of the door, around a part more distant from the center of gravity of the door on the inner peripheral surface of the base of the cylindrical part of the fixed member, and around a part closer to the center of gravity of the door on the inclined cam surface of the fixed member. As a consequence, the lower part of the axial rod and the edge of the cam surface, which are in sliding contact with these parts, having a strong external force exerted on them slantwise in the vertical direction. This external force is worse at the edge of the inclined cam surface than at the lower part of the axial rod, so that as the door rising and closing motions are repeated over a long period of time, the edge of the cam surface becomes bent, deformed by pressure, or abraded, resulting in the relatively short service life of the hinge.
In Publication No. 19394/1977, the gravity door hinge of which is of the reversible type and is applicable to either a right hand or a left hand door, the above defect is one of the greatest problems to be overcome.
In this reversible type of gravity door hinge, an inclined cam surface is formed on the inner base of a cylindrical cam inserted into a base socket of the fixed member, and also at the lower end of a cam follower inserted into the rotational member. Because the cam follower is inserted into the cylindrical cam, the bending moment generated by the off-center load occurs around a part more distant from the center of gravity of the door on the inner peripheral surface of the lower part of the cylindrical cam, and also around a part closer to the center of gravity of the door on the inner peripheral surface of the upper part of the cylindrical cam. As a consequence, the lower end of the follower, that is the edge of the inclined cam surface, and the upper end of the cam follower, which are in sliding contact with these parts, have a strong external force exerted on them slantwise in the vertical direction. This external force, similar to that in the hinge of Publication No. 1420/1951, has a relatively large and inconvenient effect on the edge of the inclined cam surface, causing the bending, deformation, or abrasion thereof. Accordingly, it is very difficult to obtain a long-lasting gravity door hinge of this type.
In the door hinge of Publication No. 19394/1977, cam accomodating the cam follower is further provided with a tapered cam surface on the inside of its base, in the center of which a small hole is bored. In addition, the center of the bottom of the socket accomodating the cam is provided with a through hole which is in communication with the small hole in the cam.
This structure proves rather disadvantageous in the following respect. When the door is at its closed position, dust and moisture in the surrounding atmosphere can enter through the through hole in the bottom of the socket into the small hole in the bottom of the cam and attach thereto. When the door is opened, the dust and moisture accumulated in the small hole advances from the small hole into the hollow space in the cam which is formed by the raising of the cam follower, and then sinks onto the cam surface inside the bottom of the cam and at the lower part inside the cam, where they finally accumulate.
This accumulation of dust is an important cause of the abrasion of the cam surfaces of the cam and cam follower and the lower part inside the cam, because the cam follower repeats its rotation and sliding in the vertical direction within the cam every time the door is opened and closed. This abrasion is very inconvenient because it reduces the life of the cam and cam follower, and also the life time of the gravity door hinge itself is reduced. The accumulated dust also works as a resistance against the rotation and vertical motion of the cam follower to prevent the smooth operation thereof.